A structure as described in Japanese Patent No. 2,888,673 (Patent Literature 1) is known as a conventional cycloidal speed reduction mechanism. The structure described in Patent Literature 1 is a cycloidal speed reduction mechanism, and can achieve a high reduction ratio as compared to planetary gear speed reduction mechanisms as typical conventional speed reducers. However, in such a conventional cycloidal speed reduction mechanism, a casing that rotatably supports an output shaft at one end in an axial direction, an internal gear that is placed in a central portion in the axial direction, and a casing that rotatably supports the output shaft at the other end in the axial direction are coupled and fixed together by bolts extending parallel to an axis. Thus, it is difficult to control assembly accuracy of the casings and the internal gear.
That is, the casings at both ends and the internal gear are assembled coaxially and in series so as to be aligned with respect to the axis. However, even slight displacement of the casings and the internal gear from the parallel arrangement after assembly is finished causes uneven wear of outer pins provided inside, thereby degrading durability performance.
For example, an in-wheel motor drive device as described in Japanese Unexamined Patent Application Publication No. 2009-52630 (Patent Literature 2) is known as an in-wheel motor drive device including a cycloidal speed reduction mechanism that has overcome this disadvantage. In the in-wheel motor drive device described in Patent Literature 2, a motor portion, a speed reduction portion that receives a driving force from the motor portion and reduces the speed of rotation to output the rotation having the reduced speed to the side of a wheel, and a wheel hub that is coupled to an output shaft of the speed reduction portion are arranged coaxially and in series. This in-wheel motor drive device is highly advantageous in that since the speed reduction portion is a cycloidal speed reduction mechanism, required torque for the drive motor can be reduced, and the size and weight of the in-wheel motor drive device can be reduced. Since a cylindrical outer-pin holding portion holds outer pins parallel to an axis, the outer pins are always held parallel, and uneven wear of the outer pins can be prevented.